cast iron water pump vs aluminum pump

Hi big moe, had the exact same problems your experiencing with a 68 Barracuda 318 I once owned, tried all the same things you did, and then I decided to haul off the intake manifold one day, was going to put on an Edelbrock. Discovered all the cooling ports on my heads where all blocked up with that rad leak additive crap, from the previous owner. Problem was fixed after clearing out all those coolant passage ways.
Not saying for sure that's the problem on your car.